The present invention relates to dc motor controllers having separately excited armature and field windings and, more particularly, to control systems without mechanical speed sensors.
Electric motors are used to power such mechanisms as appliances, trains, cranes, elevators, fork lifts, lawnmowers, passenger cars and the like. Depending on the application, the motor torque loading can be dramatically affected by terrain, loading capacity variations, etc. For these reasons and more, it is desirable to have a control system capable of extracting precise and efficient work from the appliance or vehicle. Further, it is desirable to perform such tasks prudently and economically.
In a battery-powered electrically driven vehicle, the main motive element, referred to as the traction system, conventionally consists of a series-wound dc motor typically coupled to a gear reducer and one or more drive wheels. Direction of rotation of the series-wound dc motor is controlled by the polarity orientation of the field winding with respect to the armature winding. Under conventional control, the field winding orientation is typically controlled through a pair of contactors, such that when power is applied across the field-armature combination, the motor is caused to rotate in the desired direction.
The series-wound dc motor has a very important feature: its high starting torque characteristics. However, under conventional control, the series-wound dc motor also has a major drawback. It is limited to operation along its characteristic commutation curve limit. This results in motor speed variations due to changing torque loading arising from variations in terrain, loading capacity, etc.
It is, however, possible to control the series-wound dc motor in such a fashion that the armature and field windings are separately and totally variably excited. That is to say, both the armature and field windings are independently excited, and the field is also under total variable control, as opposed to the conventional series-driven, contactor-based field control. This method of control overcomes the conventional control's aforementioned constraint of operation only along its characteristic commutation curve limit. Rather, the series-wound dc motor is now capable of being operated anywhere under its characteristic commutation curve limit. This results in improved motor controllability which is independent of motor torque loading.
The separately excited and totally variable armature and field control requires a microprocessor-based fully transistorized chopper (armature) and H-Bridge (field) control system.
Applying this separately excited and totally variable armature and field control to a series-wound dc motor results in an impractical implementation, both from a size and cost point of view, due to the typical field current levels of such motors.
To reduce packaging size and cost, it is possible to utilize a shunt-wound dc motor due to its reduced field current characteristics. However, under conventional control, where the shunt-wound dc motor's field excitation is constantly applied, the shunt-wound dc motor is incapable of providing high starting torques, and would, therefore, not be a reasonable replacement for the series-wound dc motor. But, under the separately excited and totally variable armature and field control, the shunt-wound dc motor can be made to operate with the characteristics of a series-wound dc motor, or any other motor, economically and practically.
U.S. Pat. No. 5,070,283 issued to the present inventor discloses a system for controlling a separately excited shunt-wound dc motor, where control is achieved through microprocessor-based independent pulse width modulated (PWM) frequency control of a chopper (armature) and an H-bridge (field). Connected to the armature is an armature voltage amplifier for varying the applied armature voltage. A field voltage amplifier is also provided for determining the direction of motor rotation and for varying the voltage applied to the field winding. Sensors are connected to the motor armature in order to determine the motor speed and armature current information, and to adjust the armature voltage and field voltage, so as to attain decoupling control (independent torque-speed characteristics).
U.S. Pat. No. 5,039,924 also issued to the present inventor discloses a system for controlling a separately excited shunt-wound dc motor, where control is achieved through microprocessor-based independent pulse-width-modulated/frequency control of a chopper (armature) and an H-Bridge (field). Connected to the armature is an armature voltage amplifier for varying the applied armature voltage. A field voltage amplifier is also provided for determining the direction of motor rotation and varying the voltage applied to the field winding. Sensors are connected to the motor armature and field in order to determine the motor speed, armature current, and field current information, and to adjust the armature voltage and field voltage, so as to optimize the motor's operating efficiencies.
It would be advantageous to provide a motor control system capable of producing variable torque while maintaining constant speed.
It would also be advantageous to provide a system which the characteristics of a series-wound dc motor could be simulated using a shunt-wound dc motor.
It would further be advantageous to provide a system in which a dc motor's field and armature windings are separately excited and controlled.
It would also be advantageous to provide a system in which the motor can be controlled by a decoupling controller.
It would still further be advantageous to provide a system in which the decoupling controller is achieved using software.
It would also be advantageous to provide a system in which the decoupling controller regulates motor speed without mechanical speed sensors.
It would also be advantageous to provide a system in which the dc motor's operating efficiency can be optimized without mechanical speed sensors.